How Old is Cas A?

Two
lines of evidence suggest that the Cassiopeia A remnant is about 320 years
old. The observed expansion rate and the observed size, give an estimate
of the age of about 350 years. In 1680 the British astronomer John Flamsteed
observed a star that was near the position of Cas A. This star was never
seen again, so it could well have been the explosion that produced Cas A.
This would make Cas A 319 years old. The distance to Cas A is approximately
10,000 light years, so the explosion really occurred 10,319 years ago. When
astronomers talk about such events, they are more interested in the age
of the remnant as we see it, which is important for understanding its evolution.
They take for granted that the actual event occurred earlier because of
light travel time.